1. Field of the Invention
This invention relates to silicon-on-insulator (SOI) substrates, and more particularly, to contacts to the SOI substrate.
2. Related Art
Silicon-on-insulator devices (SOI) devices are becoming the most common structure for new semiconductor designs. The SOI devices have the advantage of very excellent isolation. The structure is a silicon substrate having an overlying insulator with an overlying active layer with the devices made in the overlying active layer. The isolation completely penetrates the active layer so that the wells that are contained in a particular active area are completely isolated from other active areas. One of the difficulties with SOI devices is that the substrate does not have a convenient way to be contacted. If the substrate is left floating the substrate can influence, detrimentally, the operation of transistors in the active areas. This can cause the transistor devices to have excessive leakage, for example. In order to overcome this difficulty of a floating substrate, several techniques have been developed for making contact to it.
There are two basic approaches to making this contact. One is called backside contact, which is achieved to the backside of the semiconductor die, and the other is frontside contact, which is achieved through the side of the semiconductor die that has active circuitry. These techniques have varying types of difficulties themselves. For the frontside contact, one difficulty is having a sufficient electrical contact to the substrate. Another difficulty is that while forming an effective contact to the substrate the other devices are adversely affected.
The backside contact involves utilizing a packaging type contact in which the package itself makes contact to the backside of the silicon substrate. Although this is effective, it has also been found to be quite expensive.
Thus, there is a need for a frontside contact that avoids the problems of adversely affecting the other devices while having a sufficiently conductive contact.